Transparent conductive composite films

ABSTRACT

A process for the manufacture of a transparent conductive film comprising: (i) a polymeric substrate comprising a polymeric base layer and a polymeric binding layer, wherein the polymeric material of the base layer has a softening temperature T S-B , and the polymeric material of the binding layer has a softening temperature T S-HS ; and (ii) a conductive layer comprising a plurality of nanowires, wherein said nanowires are bound by the polymeric matrix of the binding layer such that the nanowires are dispersed at least partially in the polymeric matrix of the binding layer, said process comprising the steps of providing a polymeric substrate comprising a polymeric base layer and a polymeric binding layer; disposing said nanowires on the exposed surface of the binding layer; and heating the composite film to a temperature T 1  wherein T 1  is equal to or greater than T S-HS , and T 1  is at least about 5° C. below T S-B ; and transparent conductive films derived from said process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the national phase filing of international patentapplication No. PCT/GB2010/000937, filed 11 May 2010, and claimspriority of British patent application number GB 0908300.7, filed 14 May2009, the entireties of which applications are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention is concerned with a transparent conductivecomposite film, and an improved method for the manufacture thereof.

BACKGROUND OF THE INVENTION

Transparent conductive multi-layer films comprise a support whichexhibits high light-transmittance and/or low haze (the degree to whichlight is scattered as it passes through a material), as well as highinsulating properties, overlaid with a thin conductive layer containingan electro-conductive material. Such films must exhibit high surfaceconductivity while maintaining high optical transparency and have beenused as transparent electrodes in the manufacture of photovoltaic cells,EMI shielding screens, flat liquid crystal displays, electroluminescentdevices and touch screens in electronic equipment (for instance PDAs,mobile phones etc). Thin film photovoltaic (PV) cells are of particularinterest and for this utility the support must exhibit high lighttransmittance. Very low haze is not a requirement for supports suitablefor PV cells, and in fact a significant amount of haze can be beneficial(U.S. Pat. No. 5,078,803; Thin Solid Films 2007, 515, 8695) since itincreases the path-length of light as it travels through the PV layer.

The support can be a glass, ceramic or polymeric substrate, and recentdevelopments in flexible electronic devices have focussed on the use ofpolymeric substrates. Flexible substrates allow manufacture oftransparent conductors in a low-cost, high throughput process.Transparent conductive films composite films have typically beenproduced by vacuum deposition or sputtering techniques. Wet-coatingmethods have also been used to prepare transparent conductive films, byapplying to a substrate a coating composition comprising conductiveparticles and typically also a binder resin, which is then dried (orsintered) at high-temperature to form a conductive layer, as disclosedin for example U.S. Pat. No. 5,504,133, JP-A-8-199096, JP-A-9-109259,U.S. Pat. No. 5,908,585, U.S. Pat. No. 6,416,818, U.S. Pat. No.6,777,477, said dried layer may be then subsequently compressed asdisclosed in for example in US-2007/0145358 and US-2008/0026204.

Typically, the conductive layer comprises a conductive metal oxide suchas indium tin oxide (ITO). However, metal oxide films are fragile andprone to damage during bending or other physical stresses. They alsorequire elevated deposition temperatures and/or high annealingtemperatures to achieve high conductivity levels, and this can limitsthe applicability of vacuum deposition techniques in the manufacture offlexible electronic devices based on polymeric substrates. In addition,vacuum deposition is a costly process and requires specializedequipment, and is not conducive to forming patterns and circuits whichtypically results in the need for expensive patterning processes such asphotolithography. Conductive polymers have also been used as opticallytransparent electrical conductors, but these generally have lowerconductivity values and higher optical absorption (particularly atvisible wavelengths) compared to the metal oxide films, and can sufferfrom lack of chemical and long-term stability.

More recent developments have utilised conductive layers comprisingnanowires, as disclosed in, for instance, WO-A-2007/022226,WO-A-2008/046058, WO-A-2008/131304, WO-A-2008/147431 andWO-A-2009/017852. Typically, the nanowires are applied onto a pre-formedsubstrate and form a surface conductive network. The nanowires mustexhibit good adhesion to the substrate and good abrasion resistance. Thenanowire network is then over-coated with one or more protective orbarrier layer(s), such as an abrasion-resistant or binder layer (e.g. aUV-curable resin layer) in order to impart mechanical integrity or someother characteristic to the conductive layer, while allowing highlight-transmittance. It is believed that the network of metal nanowiresis partially embedded in the over-coat matrix such that some nanowiresmay be entirely covered by the matrix while other nanowires may protrudeabove the surface. Surface conductivity is ensured if enough protrudingnanowires percolate above the over-coat matrix.

Thus, the conventional production of transparent conductive filmscomprising nanowires involves three distinct stages: (i) the preparationof the substrate; (ii) subsequent off-line coating of the nanowires; and(iii) subsequent off-line coating of the protective over-coat layer.Typically, one or both of the off-line coating steps is effected usingsolvent-coating techniques. It would be desirable to provide a moreefficient method of manufacture, for example one which dispenses withthe over-coating step, while maintaining the mechanical integrity andabrasion resistance of the conductive layer. In addition, it would bedesirable to avoid the use of potentially hazardous and environmentallyunfriendly organic solvents in the manufacture of transparent conductivefilms.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the problemsmentioned above. It is a particular object of the invention to providean improved method of manufacture of transparent conductive films whichhaving the target electrical, optical and mechanical properties ofconventional transparent conductive films, which can be manufactured ina more efficient and economic manner.

According to the present invention, there is provided a process for themanufacture of a transparent conductive film comprising:

(i) a polymeric substrate comprising a polymeric base layer and apolymeric binding layer, wherein the polymeric material of the baselayer has a softening temperature T_(S-B); and the polymeric material ofthe binding layer has a softening temperature T_(S-HS); and(ii) a conductive layer comprising a plurality of nanowires, whereinsaid nanowires are bound by the polymeric matrix of the binding layersuch that the nanowires are dispersed at least partially in thepolymeric matrix of the binding layer,said process comprising the steps of providing a polymeric substratecomprising a polymeric base layer and a polymeric binding layer;disposing said nanowires on the exposed surface of the binding layer,preferably by dispersing said nanowires in a liquid vehicle and coatingsaid nanowire-containing liquid onto the exposed surface of the bindinglayer; and heating the composite film to a temperature T₁ wherein T₁ isequal to or greater than T_(S-HS), and T₁ is at least about 5° C.,preferably at least about 10° C., preferably at least about 20° C.,preferably at least about 30° C., and preferably at least about 50° C.,below T_(S-B).

DETAILED DESCRIPTION OF THE INVENTION

The present invention offers highly valuable improvements on theexisting manufacturing process. The process of the present inventionprovides greatly improved efficiency and significant economic benefitsto the manufacture of transparent conductive films which have thecapability to transform the development of this technology, particularlyin terms of cost.

The Polyester Substrate

The polyester substrate is a self-supporting film or sheet by which ismeant a film or sheet capable of independent existence in the absence ofa supporting base. The substrate is preferably uniaxially or biaxiallyoriented, preferably biaxially oriented. The substrate is a multilayersubstrate. The substrate may comprise one or more polymeric bindinglayer(s). Thus, the substrate may comprise a polymeric binding layer onone or both surfaces of the polymeric base layer.

The polymeric material of the binding layer should soften under heat toa sufficient extent that its viscosity becomes low enough to allowadequate wetting for it to adhere to the surface to which it is beingbonded. The polymeric material of the binding layer should soften underheat, without melting or softening of other (non-heat-sealable) layer(s)in the film. In the present invention, T_(S-HS) is at least about 5° C.below, preferably at least about 10° C. below, preferably at least about20° C. below, preferably at least about 50° C. below T_(S-B), preferablyat least about 70° C. below T_(S-B), and in one embodiment at leastabout 100° C. below T_(S-B). Preferably, T_(S-HS) is in the range offrom about 30 to about 250° C., more preferably from about 50 to about200° C., and more preferably from about 70 to about 150° C. Typically,T_(S-HS) is greater than or equal to T_(g-HS) wherein T_(g-HS) is theglass transition temperature of the polymeric material of the bindinglayer, and typically T_(S-HS) is at least about 10° C. greater thanT_(g-HS).

It will be appreciated that the multilayer polymeric substrate istypically heat-sealable, comprising a base layer and a heat-sealablelayer. Thus, the binding layer is suitably a heat-sealable layer. Thebase layer is suitably non-heat-sealable. The process for themanufacture of a transparent conductive film according to the presentinvention suitably comprises the steps of providing a polymericsubstrate comprising a polymeric base layer and a polymericheat-sealable layer. The multilayer substrate may be heat-sealable onone or both surfaces thereof.

As used herein, the term “softening temperature” is defined as theminimum temperature at which the heat-seal strength to itself of a layerin said substrate is equal to or higher than 100 g/25 mm, measured asdescribed herein.

As used herein, the term “non-heat-sealable” refers to a layer whichexhibits a heat-seal strength to itself of less than 100 g/25 mm,measured as described herein at a sealing temperature of 140° C.,particularly at a sealing temperature of 180° C., particularly at asealing temperature of 200° C., particularly at a sealing temperature of225° C., and particularly at a sealing temperature of 250° C.

The polymeric material of the binding layer typically has a degree ofcrystallinity (DOC) different to that of the base layer. Preferably, thepolymeric material of the binding layer is substantially amorphous, andpreferably has a DOC of from about 0% to about 15%, more preferably fromabout 0% to about 10%, more preferably from about 0% to about 5%,measured as defined herein. Preferably, the polymeric material of thebase layer is semi-crystalline, and preferably has a DOC of at leastabout 15%, more preferably at least about 20%, more preferably at leastabout 30%, more preferably at least about 40%, and normally no higherthan about 80%, measured as defined herein. An amorphous polymernormally begins to soften at its T_(G) or above. A semi-crystallinepolymer begins to soften only at temperatures approaching itscrystalline melting point T_(M), for example at about (T_(M)-5)° C.

The polyester(s) which constitute the substrate is/are typicallysynthetic linear polyester(s). Suitable polyesters are obtainable bycondensing one or more dicarboxylic acid(s) or their lower alkyl (up to6 carbon atoms) diesters with one or more diols. The dicarboxylic acidcomponent typically contains at least one aromatic dicarboxylic acid,which is preferably terephthalic acid, isophathalic acid, phthalic acid,1,4-, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, and is preferablyterephthalic acid or 2,6-naphthalenedicarboxylic acid. The polyester mayalso contain one or more residues derived from other dicarboxylic acidssuch as 4,4′-diphenyldicarboxylic acid, hexahydroterephthalic acid,1,10-decanedicarboxylic acid, and in particular aliphatic dicarboxylicacids including those of the general formula C_(n)H_(2n)(COOH)₂ whereinn is 2 to 8, such as succinic acid, glutaric acid sebacic acid, adipicacid, azelaic acid, suberic acid or pimelic acid, preferably sebacicacid, adipic acid and azelaic acid, and more preferably azelaic acid.The diol(s) is/are preferably selected from aliphatic and cycloaliphaticglycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol,neopentyl glycol and 1,4-cyclohexanedimethanol, preferably fromaliphatic glycols. Optionally, the diol fraction may additionallycomprise a minor proportion of one or more poly(alkylene oxide)glycol(s), typically selected from those containing C₂ to C₆ alkylenechains, and preferably polyethylene glycol (PEG). The average molecularweight of any poly(alkylene oxide) glycol used in the present inventionis typically in the range of from about 350 to about 10,000 g/mol, andit is typically present in a copolyester in no more than about 15 mol %of the glycol fraction of the copolyester, and in one embodiment in therange of from about 10 to about 15 mol % of the glycol fraction.Preferably the polyester contains only one glycol, preferably ethyleneglycol. Formation of the polyester is conveniently effected in a knownmanner by condensation or ester interchange, generally at temperaturesup to about 295° C.

The base layer preferably comprises a synthetic linear polyesterselected from those mentioned herein above, particularly a polyesterderived from one dicarboxylic acid, preferably an aromatic dicarboxylicacid, preferably terephthalic acid or naphthalenedicarboxylic acid, morepreferably terephthalic acid, and one glycol, particularly an aliphaticor cycloaliphatic glycol, preferably ethylene glycol. Polyethyleneterephthalate (PET) or polyethylene 2,6-naphthalate (PEN), particularlyPET, is the preferred polyester of the base layer. In an alternativeembodiment, the polyester is a copolyester comprising an aromaticdicarboxylic acid, preferably terephthalic acid ornaphthalenedicarboxylic acid, more preferably terephthalic acid, analiphatic glycol (preferably ethylene glycol), and a poly(alkyleneoxide) glycol (preferably (PEG)). The film-forming polymeric resin isthe major component of the base layer, and the polymeric resin makes upat least 50%, preferably at least 65%, preferably at least 80%,preferably at least 90%, and preferably at least 95% by weight of thetotal weight of the base layer.

The binding layer preferably comprises a copolyester derived from atleast two of the dicarboxylic acid(s) or their lower alkyl diesters withone or more of the glycol(s) referred to herein. The copolyester resinis the major component of the binding layer, and the copolyester makesup at least 50%, preferably at least 65%, preferably at least 80%,preferably at least 90%, and preferably at least 95% by weight of thetotal weight of the binding layer.

In one embodiment, hereinafter referred to as Embodiment A, the bindinglayer comprises a copolyester derived from one or more aliphaticglycol(s) and two or more dicarboxylic acids, preferably two or morearomatic dicarboxylic acids. Typically, the copolyester is derived froma single aliphatic glycol which in a preferred embodiment is ethyleneglycol. Preferably, the dicarboxylic acids are terephthalic acid and oneother dicarboxylic acid, preferably one other aromatic dicarboxylicacid, and preferably isophthalic acid. A preferred copolyester isderived from ethylene glycol, terephthalic acid and isophthalic acid.The preferred molar ratios of the terephthalic acid component to theisophthalic acid component are in the range of from 25:75 to 90:10,preferably 50:50 to 85:15, and in one embodiment from 50:50 to 75:25,and in a further embodiment the molar ratio is in the range from 65:35to 85:15. In a specific embodiment, this copolyester is a copolyester ofethylene glycol with about 82 mol % terephthalate and about 18 mol %isophthalate. In a further specific embodiment, this copolyester is acopolyester of ethylene glycol with about 60 mol % terephthalate andabout 40 mol % isophthalate. In a further specific embodiment, thiscopolyester is a copolyester in which the dicarboxylic acids areterephthalic acid and isophthalic acid in the aforementioned preferredmolar ratios, and the glycols are ethylene glycol and a poly(alkyleneoxide) glycol (preferably (PEG)) in the aforementioned preferred molarratios.

In a further embodiment, hereinafter referred to as Embodiment B, thebinding layer comprises a copolyester resin derived from at least one(and preferably only one) aromatic dicarboxylic acid and at least one(and preferably only one) aliphatic dicarboxylic acid (or their loweralkyl (i.e. up to 14 carbon atoms) diesters) with one or more glycol(s).A preferred aromatic dicarboxylic acid is terephthalic acid. Preferredaliphatic dicarboxylic acids are selected from sebacic acid, adipic acidand azelaic acid, particularly azelaic acid. The concentration of thearomatic dicarboxylic acid present in the copolyester is preferably nomore than about 90 mol %, preferably no more than about 80 mol %, andpreferably in the range from 45 to 80 mol %, more preferably 50 to 70mol %, and particularly 55 to 65 mol % based on the dicarboxylic acidcomponents of the copolyester. The concentration of the aliphaticdicarboxylic acid present in the copolyester is preferably at leastabout 10 mol %, preferably at least about 20 mol %, and preferably inthe range from 20 to 55, more preferably 30 to 50, and particularly 35to 45 mol % based on the dicarboxylic acid components of thecopolyester. Preferably, the glass transition temperature (T_(g-HS)) ofthe copolyester in Embodiment B is no more than about 20° C., preferablyno more than about 10° C., preferably no more than about 0° C., andpreferably no more than about −10° C. In one embodiment, the meltingpoint (T_(m)) of the copolyester of the binding layer is preferably nomore than about 160° C., preferably no more than about 150° C., and morepreferably no more than about 140° C. Particularly preferred examples ofsuch copolyesters are (i) copolyesters of azelaic acid and terephthalicacid with an aliphatic glycol, preferably ethylene glycol; (ii)copolyesters of adipic acid and terephthalic acid with an aliphaticglycol, preferably ethylene glycol; and (iii) copolyesters of sebacicacid and terephthalic acid with an aliphatic glycol, preferably butyleneglycol. Preferred polymers include a copolyester of sebacicacid/terephthalic acid/butylene glycol (preferably having the componentsin the relative molar ratios of 45-55/55-45/100, more preferably50/50/100) having a glass transition point (T_(g)) of −40° C. and amelting point (T_(m)) of 117° C.), and a copolyester of azelaicacid/terephthalic acid/ethylene glycol (preferably having the componentsin the relative molar ratios of 40-50/60-50/100, more preferably45/55/100) having a T_(g) of −15° C. and a T_(m) of 150° C.

In a further embodiment, hereinafter referred to as Embodiment C, thebinding layer comprises a copolyester derived from an aliphatic diol anda cycloaliphatic diol with one or more, preferably one, dicarboxylicacid(s), preferably an aromatic dicarboxylic acid. Examples includecopolyesters of terephthalic acid with an aliphatic diol and acycloaliphatic diol, especially ethylene glycol and1,4-cyclohexanedimethanol. The preferred molar ratios of thecycloaliphatic diol to the aliphatic diol are in the range from 10:90 to60:40, preferably in the range from 20:80 to 40:60, and more preferablyfrom 30:70 to 35:65. In a preferred embodiment this copolyester is acopolyester of terephthalic acid with about 33 mole % 1,4-cyclohexanedimethanol and about 67 mole % ethylene glycol. An example of such apolymer is PETG™6763 (Eastman) having a T_(g) of about 81° C., whichcomprises a copolyester of terephthalic acid, about 33% 1,4-cyclohexanedimethanol and about 67% ethylene glycol and which is always amorphous.In an alternative embodiment, the binding layer polymer may comprisebutane diol in place of ethylene glycol.

The thickness of the binding layer is generally between about 1 and 30%,preferably about 10 and 20% of the thickness of the substrate. Thebinding layer may have a thickness of up to about 25 μm, more preferablyup to about 20 μm, more preferably up to about 15 μm, and preferably atleast about 1 μm, more preferably at least about 2 μm, and morepreferably at least about 5 μm. The overall thickness of the substrateis preferably up to about 350 μm, more preferably up to about 200 μm,more preferably at least about 20 μm, more preferably between about 50and 150 μm.

Preferably, a binding layer of the substrate exhibits a heat-sealstrength to itself of at least 250 g/25 mm, more preferably at least 300g/25 mm, more preferably at least 400 g/25 cm, more preferably at least500 g/25 cm, more preferably at least 750 g/25 cm, more preferably atleast 1000 g/25 cm, and typically no more than about 4000 g/25 mm, moretypically no more than about 3500 g/25 mm. In one embodiment, a bindinglayer of the substrate exhibits a heat-seal strength to itself of fromabout 400 g/25 mm to about 1000 g/25 mm, and in a further embodimentfrom about 500 to about 850 g/25 mm.

Formation of the substrate may be effected by conventional extrusiontechniques well-known in the art, and in particular by co-extrusion, asdescribed below. In general terms the process comprises the steps ofextruding a layer of molten polymer, quenching the extrudate andorienting the quenched extrudate in at least one direction. Thesubstrate may be uniaxially-oriented, but is more typicallybiaxially-oriented. Orientation may be effected by any process known inthe art for producing an oriented film, for example a tubular or flatfilm process. Biaxial orientation is effected by drawing in two mutuallyperpendicular directions in the plane of the film to achieve asatisfactory combination of mechanical and physical properties. In atubular process, simultaneous biaxial orientation may be effected byextruding a thermoplastics polyester tube which is subsequentlyquenched, reheated and then expanded by internal gas pressure to inducetransverse orientation, and withdrawn at a rate which will inducelongitudinal orientation. In the preferred flat film process, thefilm-forming polyester is extruded through a slot die and rapidlyquenched upon a chilled casting drum to ensure that the polyester isquenched to the amorphous state. Orientation is then effected bystretching the quenched extrudate in at least one direction at atemperature above the glass transition temperature of the polyester.Sequential orientation may be effected by stretching a flat, quenchedextrudate firstly in one direction, usually the longitudinal direction,i.e. the forward direction through the film stretching machine, and thenin the transverse direction. Forward stretching of the extrudate isconveniently effected over a set of rotating rolls or between two pairsof nip rolls, transverse stretching then being effected in a stenterapparatus. Stretching is generally effected so that the dimension of theoriented film is from 2 to 5, more preferably 2.5 to 4.5 times itsoriginal dimension in the or each direction of stretching. Typically,stretching is effected at temperatures higher than the T_(g) of thepolyester, preferably about 15° C. higher than the T_(g). Greater drawratios (for example, up to about 8 times) may be used if orientation inonly one direction is required. It is not necessary to stretch equallyin the machine and transverse directions although this is preferred ifbalanced properties are desired.

A stretched film may be, and preferably is, dimensionally stabilised byheat-setting under dimensional support at a temperature above the glasstransition temperature of the polyester but below the meltingtemperature thereof, to induce the desired crystallisation of thepolyester. During the heat-setting, a small amount of dimensionalrelaxation may be performed in the transverse direction, TD by aprocedure known as “toe-in”. Toe-in can involve dimensional shrinkage ofthe order 2 to 4% but an analogous dimensional relaxation in the processor machine direction, MD is difficult to achieve since low line tensionsare required and film control and winding becomes problematic. Theactual heat-set temperature and time will vary depending on thecomposition of the film and its desired final thermal shrinkage butshould not be selected so as to substantially degrade the toughnessproperties of the film such as tear resistance. Within theseconstraints, a heat set temperature of about 180 to 245° C. is generallydesirable. After heat-setting the film is typically quenched rapidly inorder induce the desired crystallinity of the polyester and, inparticular, the binding layer.

In certain embodiments of the present invention, the film may be furtherstabilized through use of an on-line relaxation stage. Alternatively therelaxation treatment can be performed off-line. In this additional step,the film is heated at a temperature lower than that of the heat-settingstage, and with a much reduced MD and TD tension. The tensionexperienced by the film is a low tension and typically less than 5 kg/m,preferably less than 3.5 kg/m, more preferably in the range of from 1 toabout 2.5 kg/m, and typically in the range of 1.5 to 2 kg/m of filmwidth. For a relaxation process which controls the film speed, thereduction in film speed (and therefore the strain relaxation) istypically in the range 0 to 2.5%, preferably 0.5 to 2.0%. There is noincrease in the transverse dimension of the film during theheat-stabilisation step. The temperature to be used for the heatstabilisation step can vary depending on the desired combination ofproperties from the final film, with a higher temperature giving better,i.e. lower, residual shrinkage properties. A temperature of 135 to 250°C. is generally desirable, preferably 150 to 230° C., more preferably170 to 200° C. The duration of heating will depend on the temperatureused but is typically in the range of 10 to 40 seconds, with a durationof 20 to 30 seconds being preferred. This heat stabilisation process canbe carried out by a variety of methods, including flat and verticalconfigurations and either “off-line” as a separate process step or“in-line” as a continuation of the film manufacturing process. Film thusprocessed will exhibit a smaller thermal shrinkage than that produced inthe absence of such post heat-setting relaxation, such that theshrinkage is typically less than 1% over 30 minutes in an oven at 190°C., particularly less than 0.5%, and particularly less than 0.2%.

Formation of a multi-layer substrate comprising a binding layer and abase layer is preferably effected by co-extrusion, either bysimultaneous coextrusion of the respective film-forming layers throughindependent orifices of a multi-orifice die, and thereafter uniting thestill molten layers or, preferably, by single-channel coextrusion inwhich molten streams of the respective polymers are first united withina channel leading to a die manifold, and thereafter extruded togetherfrom the die orifice under conditions of streamline flow withoutintermixing thereby to produce a multi-layer polymeric film, which maybe oriented and heat-set as hereinbefore described. Other methods offorming the multi-layer substrate include casting the binding layer ontoa preformed base layer, and coating the binding polymer onto the baselayer, and the coating technique may be preferred for Embodiment B inparticular. Coating may be effected using any suitable coatingtechnique, including gravure roll coating, reverse roll coating, dipcoating, bead coating, extrusion-coating, melt-coating or electrostaticspray coating. Coating of the binding layer is conducted “in-line”, i.e.wherein the coating step takes place during film manufacture and before,during or between any stretching operation(s) employed. Where thebinding layer is coated, the coating step preferably avoids the use oforganic solvent, which has conventionally been used to applyheat-sealable coatings such as those of Embodiment B, and this may beachieved using the in-line processes described in WO-02/059186-A, forexample. Prior to application of a binding layer onto the base layer,the exposed surface of the base layer may, if desired, be subjected to achemical or physical surface-modifying treatment to improve the bondbetween that surface and the subsequently applied layer. For example,the exposed surface of the base layer may be subjected to a high voltageelectrical stress accompanied by corona discharge. Alternatively, thebase layer may be pre-treated with an agent known in the art to have asolvent or swelling action on the base layer, such as a halogenatedphenol dissolved in a common organic solvent e.g. a solution ofp-chloro-m-cresol, 2,4-dichlorophenol, 2,4,5- or 2,4,6-trichlorophenolor 4-chlororesorcinol in acetone or methanol.

In a preferred embodiment, however, the substrate is a multilayercoextruded substrate comprising a binding layer and a base layer.

The polyester layers in the substrate may conveniently contain any ofthe additives conventionally employed in the manufacture of polyesterfilms. Thus, agents such as cross-linking agents, dyes, pigments,voiding agents, lubricants, anti-oxidants, radical scavengers, UVabsorbers, thermal stabilisers, flame retardants and inhibitors,anti-blocking agents, surface active agents, slip aids, opticalbrighteners, gloss improvers, prodegradents, viscosity modifiers anddispersion stabilisers may be incorporated as appropriate. The film maycomprise a particulate filler which can improve handling and windabilityduring manufacture. The particulate filler may, for example, be aparticulate inorganic filler (e.g. metal or metalloid oxides, such asalumina, titania, talc and silica (especially precipitated ordiatomaceous silica and silica gels), calcined china clay and alkalinemetal salts, such as the carbonates, and sulphates of calcium andbarium). Any inorganic filler present should be finely-divided, and thevolume distributed median particle diameter (equivalent sphericaldiameter corresponding to 50% of the volume of all the particles, readon the cumulative distribution curve relating volume % to the diameterof the particles—often referred to as the “D(v,0.5)” value) thereof ispreferably in the range from 0.01 to 5 μm, more preferably 0.05 to 1.5μm, and particularly 0.15 to 1.2 μm. Preferably at least 90%, morepreferably at least 95% by volume of the inorganic filler particles arewithin the range of the volume distributed median particle diameter±0.8μm, and particularly ±0.5 μm. Particle size of the filler particles maybe measured by electron microscope, coulter counter, sedimentationanalysis and static or dynamic light scattering. Techniques based onlaser light diffraction are preferred. The median particle size may bedetermined by plotting a cumulative distribution curve representing thepercentage of particle volume below chosen particle sizes and measuringthe 50th percentile. In one embodiment, the binding layer may comprisesup to about 5% by weight (based on the total weight of the layer),preferably no more than about 2% by weight, and preferably no more thanabout 1.5% weight, of inorganic filler particles. The filler particlesare selected from the filler particles referred to hereinabove, and arepreferably selected from silica and talc, preferably silica. In thisembodiment, the windability of the film (i.e. the absence of blocking orsticking when the film is would up into a roll) is improved, without anunacceptable reduction in haze or other optical properties.

The binding layer may comprise one or more waxes, and this isparticularly appropriate where the binding layer of Embodiment B ismanufactured by coextrusion with the base-layer. Typically only one typeof wax is used. The wax may be a natural or synthetic wax, andpreferably has a melting point of at least 50° C. Natural waxes arepreferably either vegetable waxes (such as carnauba wax) or mineralwaxes (such as montan waxes and ozocerite). Paraffin waxes(highly-refined low-molecular weight waxes comprising straight-chainhydrocarbons) may also be used. Examples of synthetic waxes includeFischser-Tropsch waxes (produced by coal gasification, and having amolecular weight in the range from about 300 to about 1400 g/mol)), andoxidised and non-oxidised (preferably oxidised) low molecular weightpolyethylene waxes (having a molecular weight in the range from about500 to about 3000 g/mol) as well as the corresponding polypropylenewaxes. However, a preferred class of waxes are amide waxes. Amidic waxesare generally immiscible with the base copolyester of the binding layer.The amide wax may be a primary, secondary, tertiary or bis (fatty)amide, such as oleamide and erucamide. Examples of the different typesinclude primary fatty amides such as erucamide, behenamide, oleamide orstearamide; secondary fatty amides such as stearylerucamide,erucylerucamide, oleylpalmitamide, stearylstearamide orerucyistearamide; tertiary fatty amides such as dimethylstearamide ordiethylstearamide; and N,N′-bis (fatty) amides such as N,N′-ethylenebis(stearamide), N,N′-methylene bis(stearamide), N,N′-propylenebis(stearamide), N,N′-ethylene bis(oleamide), N,N′-methylenebis(oleamide), or N,N′-propylene bis(oleamide). Preferably, the wax isselected from N,N′-bis (fatty) amides, and more preferably fromN,N′-ethylene bis(oleamide) and N,N′-ethylene bis(stearamide). In apreferred embodiment, the wax is present at a level of from about 0.1 toabout 3 wt %, preferably from about 0.5 to about 3 wt %, preferably nomore than 2 wt %, and typically from about 1 to about 2 wt % of thetotal weight of the binding layer.

The components of the composition of a layer may be mixed together in aconventional manner. For example, by mixing with the monomeric reactantsfrom which the film-forming polyester is derived, or the components maybe mixed with the polyester by tumble or dry blending or by compoundingin an extruder, followed by cooling and, usually, comminution intogranules or chips. Masterbatching technology may also be employed.

In embodiments where the substrate comprises a base layer and a singlebinding layer, the surface of the base layer in contact with the bindinglayer is referred to herein as the primary side. The surface of the baselayer opposite to the surface which is in contact with the binding layeris referred to herein as the secondary side. The secondary side of thebase layer may have thereon one or more further polymeric layers orcoating materials. Any coating of the secondary side is preferablyperformed “in-line”. In one embodiment, the additional coating on thesecondary side may comprise a “slip coating” in order to improve thehandling and windability of the film, particularly when the base layeris a PET polyester substrate. A suitable slip coating may be, forinstance a discontinuous layer of an acrylic and/or methacrylicpolymeric resin optionally further comprise a cross-linking agent, suchas described in EP-A-0408197, the disclosure of which is incorporatedherein by reference. An alternative slip coating may comprise apotassium silicate coating, for instance as disclosed in U.S. Pat. Nos.5,925,428 and 5,882,798, the disclosures of which are incorporatedherein by reference.

The substrate exhibits a low shrinkage, and preferably less than 3% at190° C. over 30 minutes, preferably less than 2%, preferably less than1%, and preferably less than 0.5%, preferably less than 0.2%.

The substrate should be optically clear, preferably having a % ofscattered visible light (haze) of no more than 15%, preferably no morethan 10%, preferably no more than 6%, more preferably no more than 3.5%and particularly no more than 1.5%, measured according to the standardASTM D 1003. The total luminous transmittance (TLT) for light in thevisible region (400 nm to 700 nm) is preferably at least 80%, preferablyat least 85%, more preferably at least about 90%. Thus, any particulatefiller is typically present in only small amounts, generally notexceeding 0.5% and preferably less than 0.2% by weight of a given layer.

The Conductive Nanowires

The use of nanowires to form a conductive layer is known in the art, forinstance from WO-A-2007/022226, the disclosure in respect of theidentity and manufacture of nanowires is incorporated herein byreference. As used herein, the term “nanowire” refers to a conductiveelement having an aspect ratio (i.e. the length L divided by the widthW) typically in the range of 10 to 100,000. The aspect ratio is greaterthan 10, preferably greater than 50, and more preferably greater than100. At least one cross sectional dimension of the nanowire is less than500 nm, preferably less than 200 nm, and more preferably less than 100nm. The optical and electrical properties of the conductive layer aredetermined not only by aspect ratio, but also by their size, shape,distribution and density. As the diameter of the nanowire increases, theresistivity decreases substantially although it will absorb more light.For instance, overall resistivity reduces significantly as the diameterincreases from 10 nm to 100 nm, but this improvement in electricalproperties must be balanced against the decreased transparency. Whenhigh aspect ratios are used, the nanowire density required to achieve aconductive network can be low enough that the conductive network issubstantially transparent. The number of nanowires for a given densityis selected to provide acceptable electrical conduction properties. Forexample, hundreds of nanowires extending between two terminals canprovide a low resistance electrical conduction path, with theconcentration, aspect ratio, size and shape being selected to provide asubstantially transparent conductor. The distance between two electricalterminals may be such that the desired optical properties are notobtained with a single nanowire, and a plurality of many nanowires mayneed to be linked to each other at various points to provide aconductive path. Nanowire selection is normally determined by the targetoptical properties, and the number of nanowires that provides thedesired conduction path and overall resistance on that path are thenselected to achieve acceptable electrical properties for the conductivelayer. The electrical conductivity of the transparent conductive layeris mainly controlled by:

a) the conductivity of a single nanowire,

b) the number of nanowires between the terminals, and

c) the connectivity between the nanowires.

Below a certain nanowire concentration (also referred to as theelectrical percolation threshold), the conductivity between theterminals is zero, i.e. there is no continuous current path providedbecause the nanowires are spaced too far apart. Above thisconcentration, there is at least one current path available. As morecurrent paths are provided, the overall resistance of the layerdecreases.

Conductive nanowires include metal nanowires and other conductiveparticles having high aspect ratios (i.e. higher than 10). Examples ofnon-metallic nanowires include, but are not limited to, carbon nanotubes(CNTs), conductive polymer fibers and the like. In one advantageousembodiment, the nanowires are metal nanowires. As used herein, the term“metal nanowire” refers to a nanowire comprising elemental metal, metalalloys or metal compounds (including metal oxides). Metals, metal alloysand metal oxides that can be used as nanowires include, withoutlimitation, Cu, Au, Ag, Ni, Pd, Co, Pt, Ru, W, Cr, Mo, Ag, Co alloys(e.g. CoPt), Ni alloys, Fe alloys (e.g. FePt) Or TiO₂, CO₃O₄, Cu₂O,HfO₂, ZnO, vanadium oxides, indium oxide, aluminum oxide, indium tinoxide, nickel oxide, copper oxide, tin oxide, tantalum oxide, niobiumoxide, vanadium oxide or zirconium oxide. Suitable metal nanowires canbe based on any metal, and of particular utility are silver, gold,copper, nickel, and gold-plated silver. Metal nanowires can be preparedby methods known in the art. For instance, silver nanowires can besynthesized through solution-phase reduction of a silver salt (e.g.,silver nitrate) in the presence of a polyol (e.g., ethylene glycol) andpolyvinyl pyrrolidone), as reported in Nano Lett. 2002, 2, 165.Alternatively, metal nanowires can be prepared using biologicalmaterials as templates, including for instance proteins, peptides,phages, bacteria, viruses, and the like, as discussed inWO-A-2007/022226. The use of biological template allows for theselective formation of conductive layers having a highly connectednetwork than would be possible with random nanowires, as well asselective formation of nanowires having particular dimensions,morphologies and compositions.

A conductive layer of nanowires comprises a (typically sparse) networkof nanowires. As used herein, the term “conductive layer” refers to thenetwork of nanowires that provide the conductive media of thetransparent conductive composite films. Conductivity is normallyachieved by electrical charge percolation from one nanowire to another,thereby requiring a sufficient amount of nanowires to be present in theconductive layer to reach an electrical percolation threshold and becomeconductive. The amount (also referred to as the “threshold loadinglevel”) of nanowires required to achieve the desired sheet resistancedepends on factors such as the aspect ratio, the degree of alignment,degree of agglomeration and the resistivity of the nanowires. In thecase of silver nanowires, for example, high aspect ratios allow for theformation of a conductive network at a threshold surface loading levelwhich is preferably in the range from about 0.05 μg/cm² to about 10μg/cm², more preferably from about 0.1 μg/cm² to about 5 μg/cm² and morepreferably from about 0.8 μg/cm² to about 3 μg/cm². Higher loadinglevels tend to compromise the mechanical or optical properties of thepolymeric matrix. The precise loading levels depend strongly on thedimensions and spatial dispersion of the nanowires. Advantageously,transparent conductors of tunable electrical conductivity and opticaltransparency can be provided by adjusting the loading levels of themetal nanowires.

In the present invention, the nanowires are held partially or completelywithin the polymeric matrix of the binding layer of the substrate. Thus,the “conductive layer” defined by the nanowire network may, at leastpartially, occupy the same space as the binding layer. Portions of thenanowire network may protrude from the polymeric matrix, for instance toenable electrical connection to the conductive network. The polymericmatrix of the binding layer advantageously protects the nanowire networkfrom adverse environmental factors, such as corrosion and abrasion, andprovides the conductive layer with desirable physical and mechanicalproperties, including adhesion to the substrate, strength andflexibility.

In one embodiment, the conductive layer spans the entire thickness ofthe polymeric matrix of the binding layer. Advantageously, a portion ofthe nanowires may be exposed on a surface of the polymeric matrix, whichis particularly useful for touch screen applications. In particular, atransparent conductor can display surface conductivity on at least onesurface thereof. While some nanowires may be entirely submerged in thematrix, other nanowires protrude above the surface. If enough nanowiresprotrude above the matrix, then the surface of the transparent conductorbecomes conductive.

In an alternative embodiment, the conductive layer formed by thenanowire network is entirely submerged in a portion of the polymericmatrix of the binding layer.

The nanowires may be used in association with corrosion inhibitors, astaught in WO-A-2007/022226, which should be soluble or miscible andotherwise compatible with the polyester(s) of the substrate layer, andin particular with the copolyester(s) of the binding layer.

The Composite Film and Manufacture Thereof

Application of the conductive layer to the substrate is typicallyeffected by dispersing the nanowires into a liquid vehicle, and thencoating the composition onto the surface of the binding layer of thesubstrate. Any suitable liquid vehicle may be used, including organicsolvents (such as alcohols, ketones, ethers or volatile hydrocarbons),but preferably the coating composition is an aqueous dispersion. Theliquid vehicle may optionally contain additives such as viscositymodifiers, surfactants, corrosion inhibitors and the like, as is knownin the art. For instance, a nanowire dispersion may comprise, by weight:

(i) from 0.0025% to 0.1% surfactant (for example Zonyl® FSO-100);

(ii) from 0.02% to 4% viscosity modifier (for example, hydroxyl propylmethyl cellulose);

(iii) from 0.05% to 1.4% metal nanowires (for example, silver nanowires;and

(iv) from 94.5% to 99.0% solvent (for example, water or isopropanol).

The weight ratio of the surfactant to the viscosity modifier ispreferably in the range of about 80:1 to about 0.01:1. The weight ratioof the viscosity modifier to the nanowires is preferably in the range ofabout 5:1 to about 0.000625:1. The ratio of the metal nanowires to thesurfactant is preferably in the range of about 560:1 to about 5:1. Theviscosity of the nanowire dispersion is preferably in the range of fromabout 1 to about 100 cP.

Coating may be effected using any suitable coating technique, includinggravure roll coating, reverse roll coating, dip coating, bead coating,extrusion-coating, melt-coating or electrostatic spray coating.

The conductive layer may be applied to the surface of the binding layerof the substrate after the film manufacture process has been completed(referred to herein as “off-line” application), i.e. after theheat-setting and optional heat-stabilisation steps described hereinabovehave been conducted. Following deposition of the nanowires, the film isheated to a temperature T₁, as defined hereinabove. In this embodiment,there is no need for the additional step of over-coating the depositednanowires with an abrasion-resistant or binder layer, thereby improvingthe efficiency of the manufacturing process.

In a preferred embodiment, however, the conductive layer is applied tothe surface of the binding layer of the substrate during the filmmanufacturing process (referred to herein as “in-line” application), andin particular prior to the heat-setting step. In this embodiment, thenanowires may be applied to the surface of the binding layer of thesubstrate before or after the stretching steps have been completed, butpreferably between the two stages (longitudinal and transverse) of thebiaxial stretching operation described hereinabove. Following depositionof the nanowires, the film is heated to a temperature T₁, as definedhereinabove, and this is preferably effected by the heat-setting stepconventionally used in the film manufacturing process describedhereinabove. In this preferred embodiment, not only is there no need forthe additional step of over-coating the deposited nanowires with anabrasion-resistant or binder layer, but there is also no need for aseparate off-line step of deposition of the nanowires. The production ofthe transparent conductive film is achieved in a single pass, with thedeposition and binding of the nanowires being effected during filmmanufacture, thereby greatly improving the efficiency of themanufacturing process.

In either of the process embodiments described immediately above, thebinding layer may be coated onto the base layer, although preferably thebase layer and the binding layers are coextruded.

The heating temperature T₁ is typically at least about 50° C., moretypically at least about 80° C. and typically less than about 240° C.,typically less than about 220° C., and typically less than about 200° C.The duration of the heating step is preferably in the range of fromabout 10 seconds to about 5 minutes, preferably from about 20 seconds toabout 3 minutes and typically from about 30 seconds to about 60 seconds.The heating of the coated substrate causes the nanowires to sink atleast partially into the polymeric matrix of the binding layer. In oneembodiment, the nanowires sink below the surface of the binding layer sothat they are fully submerged within the layer.

The nanowire network may require additional processing steps to renderit electrically conductive, for instance exposure to plasma, coronadischarge, UV-ozone, or pressure.

In one embodiment, the composite film comprising substrate andconductive layer is optionally compressed, either during or subsequentto the heating step, and this may assist in increasing the conductivityof the nanowire network layer. The compression force is preferably atleast 44 N/mm², more preferably at least 135 N/mm², more preferably atleast 180 N/mm², but typically no more than 1000 N/mm². The compressioncan be effected using conventional methods, including sheet pressing androll pressing, and preferably by roll pressing in which the film to becompressed is held between rolls and compressed as the rolls rotate,thereby allowing roll-to-roll production.

The conductive layer may be formed on both sides of a substratecomprising a base layer and on each surface thereof a binding layer, thesame or different.

The nanowires can be deposited in a pre-determined pattern. Thus, priorto deposition of the nanowires, the surface of the binding layer of thesubstrate may be pre-treated in accordance with a pre-determinedpattern, for instance using a plasma surface treatment carried outthrough an aperture mask having the desired pattern. Nanowires depositedon the pre-treated region of the surface exhibit greater adhesionrelative to the untreated region and, accordingly, patterned depositioncan be achieved by removing the nanowires on the untreated region by anappropriate method (such as washing with a suitable solvent, brushing orby transferring them to a tacky or adhesive roller). Alternatively, aroller (e.g. a Gravure roller) or stamp having recesses of apredetermined pattern can be used to coat the nanowire dispersion,thereby enabling patterned deposition and production of a patternedconductive layer. The conductive layer can also be patterned by sprayingthe nanowire onto the substrate through an aperture mask.

The composite film and process of the present invention are particularlyadvantageous because they provide the nanowire network with sufficientmechanical strength and abrasion resistance without the need for anadditional over-coating step in which the nanowire-coated substrate iscoated with an additional protective layer (for instance furtherpolymeric material), as hitherto required for the manufacture ofnanowire-containing transparent conductive films of practical utility.Thus, the process of the present invention avoids the need for, andexcludes the step of, over-coating said deposited nanowires with anovercoat matrix, for instance a barrier or protective layer (such as anabrasion-resistant or binder layer), for instance where the nanowirenetwork becomes at least partially embedded in said overcoat matrix as aresult of said over-coating step.

The transparent conductive film produced according to the inventiondescribed herein preferably exhibits a sheet resistance of no more thanabout 10⁶ Ω/square, preferably no more than about 100,000 Ω/square,preferably no more than about 50,000 Ω/square preferably no more thanabout 10,000 Ω/square, preferably no more than about 1,000 Ω/square,preferably no more than 750 Ω/square, preferably no more than 500Ω/square, preferably no more than 250 Ω/square, and most preferably nomore than 100 Ω/square, and is typically at least 1 Ω/square.

In one embodiment, the composite film comprising polymeric substrate andconductive layer preferably exhibits a % of scattered visible light(haze) of no more than 50%, preferably no more than 40%, preferably nomore than 30%, preferably no more than 25%, preferably no more than 20%,preferably no more than 15%, preferably no more than 10%, preferably nomore than 5%, measured according to the standard ASTM D 1003. The totalluminous transmittance (TLT) for light in the visible region (400 nm to700 nm) is preferably at least 65%, preferably at least 75%, morepreferably at least 80%, more preferably at least 85%, more preferablyat least about 90%. In one embodiment, for instance where the conductivelayer comprises metal nanowires, the haze is preferably no more thanabout 15% and/or the TLT is at least 80%.

According to a further aspect of the present invention, there isprovided a transparent conductive film comprising:

(i) a polymeric substrate comprising a polymeric base layer and apolymeric binding layer; and

(ii) a conductive layer comprising a plurality of nanowires, whereinsaid nanowires are bound by the polymeric matrix of the binding layersuch that the nanowires are dispersed at least partially in thepolymeric matrix of the binding layer,

particularly such a film obtained by the process as defined in claim 1.

The transparent conductive films made according to the process describedherein exhibit the following properties:

-   -   (i) low sheet resistance;    -   (ii) high light transmittance;    -   (iii) low haze;    -   (iv) excellent adhesion of the nanowires to the substrate; and    -   (v) good abrasion resistance of the nanowires,        which are at least comparable with transparent conductive films        made according to a conventional process, or which satisfy the        threshold levels for that property required by a commercial        transparent conductive film. Thus, it will be appreciated that        the primary objective of the invention is an improved process        for the manufacture of transparent conductive films exhibiting        properties which are commercially acceptable and/or comparable        to those of the prior art, rather than seeking to improve those        properties per se. Some of the films produced according to the        present invention might be inferior in certain properties (for        instance, abrasion resistance or haze) when compared with the        best prior art films, but all have commercial utility and, more        importantly, all are produced via a process which advantageously        offers significant improvements in efficiency and economy.

According to a further aspect of the invention, there is provided theuse of a polymeric substrate comprising a polymeric base layer and apolymeric binding layer as a substrate in the manufacture of atransparent conductive film comprising a conductive layer comprising aplurality of nanowires.

Property Measurement

The following analyses were used to characterize the films describedherein:

-   (i) Optical Properties were evaluated by measuring total luminance    transmittance (TLT) for light in the visible region (400 nm to 700    nm) and haze (% of scattered transmitted visible light) through the    total thickness of the film using an M57D spherical hazemeter    (Diffusion Systems) according to the standard test method ASTM    D1003.-   (ii) Sheet resistance (ohms/square or Ω/sq) of the conductive layer    was measured using a linear four point probe (Jandel Model RM2)    according to ASTM F390-98 (2003).-   (iii) Thermal shrinkage was assessed for film samples of dimensions    200 mm×10 mm which were cut in specific directions relative to the    machine and transverse directions of the film and marked for visual    measurement. The longer dimension of the sample (i.e. the 200 mm    dimension) corresponds to the film direction for which shrinkage is    being tested, i.e. for the assessment of shrinkage in the machine    direction, the 200 mm dimension of the test sample is oriented along    the machine direction of the film. After heating the specimen to the    predetermined temperature (by placing in a heated oven at that    temperature) and holding for an interval of 30 minutes, it was    cooled to room temperature and its dimensions re-measured manually.    The thermal shrinkage was calculated and expressed as a percentage    of the original length.-   (iv) Heat-seal strength to itself of a surface of the substrate    (including the heat-sealable or binding layers) is measured in an    Instron Model 4301 by positioning together the surfaces of two 25 mm    wide samples of polyester film and heating the laminate structure at    140° C. for two seconds under a pressure of 0.1 MPa. The sealed film    is cooled to room temperature, and the heat-seal strength determined    by measuring the force required under linear tension per unit width    of seal to peel the layers of the film apart at a constant speed of    4.23 mm/second. It will be appreciated that the heat-seal strength    is provided by, and is a property of, the polymeric material of the    substrate surface and is measured as such in the absence of    nanowires. Thus, in the case of off-line nanowire application as    described herein, heat-seal strength is measured on the substrate    prior to application of the nanowires. In the case of in-line    nanowire application as described herein, heat-seal strength is    typically measured on the substrate of the finished film (i.e.    post-stenter) during the film production run but before or after    (but normally before) application of the nanowires, for instance    during the quality-control phase at the start of a polyester film    manufacturing run.-   (v) Softening temperature is measured by assessing the heat-seal    strength to itself of a layer in the substrate to itself over a    range of temperatures, typically 80 to 200° C. The softening    temperature is the minimum temperature at which the heat-seal    strength is equal to or higher than 100 g/25 mm. The heat-seal    strength is measured as described hereinabove, but with variation of    the sealing temperature.-   (vi) Abrasion resistance of the composite film was evaluated using a    Crockmeter from Atlas Electric Devices Co. After 50 crocks (one    “crock” is one forward and one backward rub with a 2×2 cm² dry    cloth) the film surface was inspected visually and graded with a    grade of 1 to 5, where grade 1 corresponds to no visible scratching    of the conductive surface, grade 2 corresponds to up to about 20%    visible scratching, grade 3 corresponds to up to about 50% visible    scratching, grade 4 corresponds to up to about 80% visible    scratching and grade 5 corresponds to at least about 81% scratching,    all respective to the conductive layer. The abrasion resistance test    is a particularly harsh test designed to assess the performance of    the film at extremes not normally experienced in the potential    end-use. It will therefore be appreciated that a high percentage of    scratching doesn't necessarily preclude a given film from commercial    utility.-   (vi) Adhesion of the nanowires to the substrate was evaluated using    4104-grade tape from Tesa (2.5 cm wide), either alone or with a    cross-hatch tool. The Tesa tape was applied to the surface of the    film sample at room temperature and a spatula was used to smooth    down the tape, ensuring good contact with the film. The tape was    then removed using a fast pull by hand. The results of the    tape-alone adhesion were recorded as either “pass” (i.e. no coating    removed) or “fail” (i.e. some or all coating removed). The results    of the cross-hatch adhesion test were recorded as a percentage of    the nanowire network retained on the surface. Films according to the    invention exhibit a retention level of at least 90%, preferably at    least 95%, preferably at 99% and preferably substantially complete    retention.-   (viii) The degree of crystallinity (DOC) of a polymer sample is the    fractional amount of crystallinity in a polymer sample, and relies    on the assumption that the sample can be sub-divided into a    crystalline phase and an amorphous phase (the Two-Phase Model)    wherein each phase has properties identical with those of their    ideal states, with no influence of interfaces. The degree of    crystallinity of a polyester resin can be measured via measurement    of density, and applying the following relationship:    V _(c)=(P−P _(a))·(P _(c) −P _(a))⁻¹-    wherein V_(c)=volume fraction crystallinity; P=density of sample;    P_(a)=density of amorphous material; and P_(c)=density of    crystalline material. The density P can be measured in a density    column, for instance using n-heptane/carbon tetrachloride mixtures.

The invention is further illustrated by the following examples. Theexamples are not intended to limit the invention as described above.Modification of detail may be made without departing from the scope ofthe invention.

EXAMPLES Example 1

A composite film comprising a base layer of clear PET and aheat-sealable (binding) layer comprising a copolyester of terephthalicacid/isophthalic acid/ethylene glycol (82/18/100 molar ratio) wasprepared by coextrusion. The polymer layers were coextuded usingseparate streams supplied from separate extruders, to a single channelcoextrusion assembly. The polymer layers were extruded through afilm-forming die on to a water-cooled rotating, quenching drum atvarious line speeds to yield an amorphous cast composite extrudate. Thecast extrudate was heated to a temperature in the range of about 50 to80° C. and then stretched longitudinally at a forward draw ratio ofabout 3:1. The polymeric film was passed into a stenter oven at atemperature of about 100° C., where the sheet was stretched in thesideways direction to approximately 3 times its original dimensions, andthen heat-set at a temperature of about 230° C. The heat-set film wasthen quenched rapidly in air at a temperature of about 25° C. The totalthickness of the final film was 100 μm. The amorphous heat-sealable(binding) layer was approximately 15 μm thick, and exhibited a degree ofcrystallinity of less than 5%. The semi-crystalline base layer exhibiteda degree of crystallinity of about 45%. The polymeric material of theheat-sealable (binding) layer exhibited a softening temperature of about119° C. The polymeric material of the base layer exhibited a meltingpoint of about 250° C. The film was transparent with a haze of 1.4% anda TLT of 90.7%. The heat-seal strength to itself of the binding layer ofthe film was about 3150 g/25 mm.

The film was then coated on its heat-sealable surface (i.e. on thebinding layer) with an aqueous dispersion of silver nanowires(approximately 0.2 wt % silver) which had been filtered through a 75 μmmesh. The coating was performed using a No. 4 Meyer bar, to apply alayer having a wet-coat thickness of about 36 μm. Subsequently, samplesof the coated film were dried at various temperatures (140, 160, 180 and200° C.) for either 30 or 60 seconds. The characteristics of the filmwere measured as described herein and the results presented in Table 1abelow.

Comparative Example 1

A polymer composition comprising polyethylene terephthalate was extrudedand cast onto a cooled rotating drum and stretched in the direction ofextrusion to approximately 3 times its original dimensions. The film waspassed into a stenter oven at a temperature of 100° C. where the filmwas stretched in the sideways direction to approximately 3 times itsoriginal dimensions. The biaxially stretched film was heat-set at about230° C. by conventional means. The total thickness of the final film was125 μm. The film was transparent with a haze of 0.5% and a TLT of 90.0%.The semi-crystalline PET layer exhibited a degree of crystallinity ofabout 45%, and a melting point of about 250° C. The film wasnon-heat-sealable. Samples of the film were coated with silver nanowiresand dried, in the same way as for Example 1. The characterising data arein Table 1a.

TABLE 1a Adhesion Adhesion Temp Time SR TLT Haze (tape) (cross-hatch)Sample (° C.) (s) (Ω/sq) (%) (%) (Pass/Fail) (%) C. Ex. 1 140 30 41 87.04.1 Fail 12 C. Ex. 1 140 60 22 86.8 4.4 Fail 50 Ex. 1 140 30 66 87.6 4.8Pass 100 Ex. 1 140 60 29 87.2 6.3 Pass 100 C. Ex. 1 160 30 29 86.8 4.5Fail 0 C. Ex. 1 160 60 27 86.7 4.3 Fail 4 Ex. 1 160 30 63 87.3 5.3 Pass100 Ex. 1 160 60 25 87.2 6.7 Pass 100 C. Ex. 1 180 30 25 86.7 4.0 Fail40 C. Ex. 1 180 60 24 86.7 4.8 Fail 53 Ex. 1 180 30 32 87.3 6.4 Pass 100Ex. 1 180 60 30 87.0 6.8 Pass 100 C. Ex. 1 200 30 24 86.8 4.4 Fail 8 C.Ex. 1 200 60 22 86.4 6.2 Fail 24 Ex. 1 200 30 28 87.1 6.9 Pass 100 Ex. 1200 60 32 86.8 7.6 Pass 100

The experiments show that the non-heat-sealable PET film exhibited poorperformance in the adhesion test, whereas the heat-sealable filmexhibited excellent results. The data also show that the haze generallyincreases with increased temperatures and duration of the heating step,without adverse affect to the sheet resistance. The abrasion resistanceof all samples in Table 1a was scored as 5 in the test described herein.

Comparative Example 2

A polymer composition comprising polyethylene terephthalate was extrudedand cast onto a cooled rotating drum and stretched in the direction ofextrusion to approximately 3 times its original dimensions. The film waspassed into a stenter oven at a temperature of 100° C. where the filmwas stretched in the sideways direction to approximately 3 times itsoriginal dimensions. The biaxially stretched film was heat-set at about230° C. by conventional means. The total thickness of the final film was125 μm. The film was transparent with a haze of 0.5% and a TLT of 90.0%.The semi-crystalline PET layer exhibited a degree of crystallinity ofabout 45%, and a melting point of about 250° C. The film was coated witha silver nanowire dispersion using a No. 5 Meyer bar to apply a layerhaving a wet-coat thickness of about 50 μm and dried, in the same way asfor Example 1. A UV-curable overcoat (3% solids in MEK) was subsequentlyapplied to the silver nanowire-coated film using a No. 2 Meyer bar(about 12 μm wet coat thickness). The coated sample was dried in an ovenset at 70° C. for 1 minute and cured through a UV curer. The over-coatedconductive film exhibited a sheet resistance of 15 Ω/square; a TLT of87.5%; and a haze of 5.3%. It passed the adhesion tape test and scored100% at the cross-hatch adhesion test. The abrasion resistance wasscored as 5 in the test described herein.

Thus, it will be appreciated that the adhesion and abrasion resistanceof the heat-sealable film of Example 1 made according to theadvantageous process of the invention is comparable with that of theover-coated conductive film of Comparative Example 2 made according tothe conventional process.

Example 2 and Comparative Example 3

Example 1 and Comparative Example 1 were repeated except that instead ofcoating with silver nanowires, the polyester films were coated with anaqueous dispersion of carbon nanotubes (total solids content<5 wt %, andapproximately 1 wt % carbon nanotubes). The coating was performed usinga No. 3 Meyer bar, to apply a layer having a wet-coat thickness of about24 μm. Subsequently, the coated films were dried at various temperatures(140, 160, 180, 200, 220 and 240° C.) for either 30 or 60 seconds. Thecharacteristics of the film were measured as described herein and theresults presented in Table 1b hereinbelow.

TABLE 1b Adhesion Abrasion Temp Time SR TLT Haze (tape) resistanceSample (° C.) (s) (Ω/sq) (%) (%) (Pass/Fail) (1-5) C. Ex. 3 140 30 1950070.0 5.4 Fail 5 C. Ex. 3 140 60 16900 71.4 5.0 Fail 5 Ex. 2 140 30 2230069.0 5.0 Pass 5 Ex. 2 140 60 24000 69.9 4.6 Pass 5 C. Ex. 3 160 30 1800073.5 5.4 Fail 5 C. Ex. 3 160 60 14800 68.6 5.1 Fail 5 Ex. 2 160 30 2490072.9 3.8 Pass 5 Ex. 2 160 60 21100 69.7 4.7 Pass 4 C. Ex. 3 180 30 1340067.2 5.2 Fail 5 C. Ex. 3 180 60 14600 69.2 5.2 Fail 5 Ex. 2 180 30 2200069.5 4.1 Pass 5 Ex. 2 180 60 21200 69.3 4.4 Pass 5 C. Ex. 3 200 30 1380068.2 5.1 Fail 5 C. Ex. 3 200 60 11900 65.3 6.0 Fail 5 Ex. 2 200 30 2020067.7 4.4 Pass 4 Ex. 2 200 60 20800 69.7 5.1 Pass 3 C. Ex. 3 220 30 1330069.6 5.0 Fail 5 C. Ex. 3 220 60 12200 70.5 5.0 Fail 5 Ex. 2 220 30 2280070.9 4.2 Pass 3 Ex. 2 220 60 25900 71.4 5.4 Pass 3 C. Ex. 3 240 30 1300068.3 5.2 Fail 5 C. Ex. 3 240 60 12800 65.4 6.3 Fail 5 Ex. 2 240 30 2840071.1 2.7 Pass 3 Ex. 2 240 60 25400 69.4 4.9 Pass 3

The experiments show that the non-heat-sealable PET film exhibited poorperformance in the adhesion test, whereas the heat-sealable filmexhibited excellent adhesion results.

Example 3

Example 1 was repeated except that the heat-sealable surface of thecoextruded film was coated with an aqueous dispersion of silvernanowires (as described above) which had been filtered through a 10 μmmesh. The coating was performed using a No. 5 Meyer bar to apply a layerhaving a wet-coat thickness of about 50 μm and samples of the coatedfilm were then dried at various temperatures (180, 200, 220 and 240° C.)for either 30 or 60 seconds. The characterising data are presented inTable 2 hereinbelow.

Example 4

Example 3 was repeated except that the PET base layer contained 1500 ppmchina clay and the final film thickness was 50 μm, with a heat-sealable(binding) layer which was approximately 10 μm thick. The polymericmaterial of the heat-sealable layer exhibited a softening temperature of100° C. The film was transparent with a haze of 9.3% and a TLT of 87.4%.The heat-seal strength to itself of the binding layer of the film wasabout 1800 g/25 mm.

Example 5

Example 3 was repeated except that two layers of terephthalicacid/isophthalic acid/ethylene glycol (82/18/100 mol %) copolyester werecoextruded either side of the PET base layer and one of the sides wascoated with an adhesion primer during film manufacture (the nano-wireswere subsequently applied onto the non-primed side). The final filmthickness was 105 μm, with each heat-sealable (binding) layer beingapproximately 18 μm thick. The polymeric material of the heat-sealable(binding) layer exhibited a softening temperature of 118° C. The filmwas transparent with a haze of 1.3% and a TLT of 90.8%. The heat-sealstrength to itself of the binding layer of the film was about 3200 g/25mm.

TABLE 2 Abrasion Temp Time SR TLT Haze Resistance Sample (° C.) (s)(Ω/sq) (%) (%) (1-5) Ex. 3 180 30 18 85.8 7.5 5 Ex. 4 180 30 20 84.115.5 5 Ex. 5 180 30 33 86.6 6.4 5 Ex. 3 180 60 18 85.7 9.5 4 Ex. 4 18060 19 83.5 17.0 3 Ex. 5 180 60 19 85.6 10.4 3 Ex. 3 200 30 16 84.8 9.0 5Ex. 4 200 30 17 83.0 18.3 5 Ex. 5 200 30 19 86.6 8.2 5 Ex. 3 200 60 1785.7 12.8 4 Ex. 4 200 60 18 83.1 19.9 3 Ex. 5 200 60 18 85.6 11.9 3 Ex.3 220 30 17 85.3 13.9 4 Ex. 4 220 30 19 81.3 23.7 3 Ex. 5 220 30 18 86.011.6 4 Ex. 3 220 60 19 85.6 12.4 3 Ex. 4 220 60 19 82.3 24.2 2 Ex. 5 22060 19 85.4 13.8 3 Ex. 3 240 30 19 84.5 19.5 3 Ex. 4 240 30 28 82.4 42.62 Ex. 5 240 30 18 85.2 17.8 3 Ex. 3 240 60 29 82.7 11.9 2 Ex. 4 240 60122 81.8 16.2 2 Ex. 5 240 60 47 85.2 7.9 2

The results demonstrate that the abrasion resistance is comparable to orbetter than the over-coated films. The results also demonstrate thatabrasion resistance can be correlated to the drying time andtemperature.

Example 6

A composite film was manufactured by coextrusion in which the first(base) layer was unfilled polyethylene terephthalate (PET), and thesecond (binding) layer was a heat-sealable copolyester of azelaicacid/terephthalic acid/ethylene glycol (45/55/100 molar ratio) having aT_(g) of −15° C. and a T_(m) of 150° C. The heat-sealable layer furthercomprised 1.5% by weight (relative to the total composition of thelayer) of an N,N′-ethylene bis(oleamide) wax (EBO; obtained as CrodamideEBO from Croda), and 3% by weight (relative to the total composition ofthe layer) of silica filler particles with an average particle size of 1μm.

The coextrusion was effected generally in accordance with Example 1,except that the temperature of the transverse stretch was about 110° C.,the transverse stretch ratio was about 4, and the heat-set temperaturewas between about 210 and 225° C. The final thickness of the film was 25μm, in which the heat-sealable layer was 0.7 μm in thickness. The filmwas transparent with a haze of 6%. The heat-seal strength to itself ofthe binding layer of the film was about 500 g/25 mm.

A nanowire layer was applied to the film in the manner described above,and heated at 180, 200, 220 and 240° C. for either 30 or 60 seconds. Allfilms passed the adhesion test described herein and exhibited 100%adhesion in the cross-hatch test.

Example 7a

A composite film comprising a base layer of a copolyester ofterephthalic acid/ethylene glycol/PEG (100/88/12 molar ratio; PEGmolecular weight was 3450 g/mol; china clay filler present at 2100 ppmby weight) and a heat-sealable (binding) layer comprising a copolyesterof terephthalic acid/isophthalic acid/ethylene glycol/PEG (82/18/88/12molar ratio; PEG molecular weight was 400 g/mol; china clay fillerpresent at 2100 ppm by weight) was prepared by coextrusion. The polymerlayers were coextuded using separate streams supplied from separateextruders, to a single channel coextrusion assembly. The polymer layerswere extruded through a film-forming die on to a water-cooled rotating,quenching drum at various line speeds to yield an amorphous castcomposite extrudate. The cast extrudate was heated to a temperature inthe range of about 50 to 80° C. and then stretched longitudinally at aforward draw ratio of about 3:1. The polymeric film was passed into astenter oven at a temperature of about 100° C., where the sheet wasstretched in the sideways direction to approximately 3 times itsoriginal dimensions, and then heat-set at temperatures between 190 and210° C. for about 1 minute. The heat-set film was then quenched rapidlyin air at a temperature of about 25° C. The total thickness of the finalfilm was 23 μm. The amorphous heat-sealable layer was approximately 2 μmthick, and exhibited a degree of crystallinity of less than about 5%.The semi-crystalline base layer exhibited a degree of crystallinity ofabout 35%. The polymeric material of the heat-sealable layer exhibited asoftening temperature of about 81° C. The polymeric material of the baselayer exhibited a melting point of about 250° C. The film wastransparent with a haze of 5.1% and a TLT of 88.4%. The heat-sealstrength to itself of the binding layer of the film was about 680 g/25mm.

Example 7b

The procedure of Example 7a was repeated except that the film wasin-line coated on its heat-sealable surface with an aqueous dispersionof silver nanowires (approximately 0.2 wt % silver) which had beenfiltered through a 75 μm mesh. The coating step was effected after thefilm was stretched longitudinally and before it was passed into thestenter oven. The coating was applied at a wet coat weight between 36and 100 μm. The characteristics of the film were measured as describedherein and the results presented in Table 3 below.

TABLE 3 Adhesion Adhesion Wet Coat SR TLT Haze (tape) (cross-hatch)Sample (μm) (Ω/sq) (%) (%) (Pass/Fail) (%) Ex. 7b-i 36 500 84.5 11.5Pass 100 Ex. 7b-ii 50 100 82.2 14.3 Pass 100 Ex. 7b-iii 60 82 81.7 14.8Pass 100 Ex. 7b-iv 100 14 77.3 21.9 Pass 100

The invention claimed is:
 1. A process for the manufacture of atransparent conductive film comprising: (i) a polymeric substratecomprising a polyester base layer and a polyester binding layer, whereinthe polymeric material of the base layer has a softening temperatureT_(S-B), and the polymeric material of the binding layer has a softeningtemperature T_(S-HS); and (ii) a conductive layer comprising a pluralityof nanowires, wherein: said nanowires are bound by the polymeric matrixof the binding layer such that the nanowires are dispersed at leastpartially in the polymeric matrix of the binding layer, said processcomprising the steps of providing a polymeric substrate comprising apolyester base layer and a polyester binding layer; disposing saidnanowires on the exposed surface of the binding layer; and heating thecomposite film to a temperature T₁ wherein T₁ is equal to or greaterthan T_(S-HS), and T₁ is at least about 5° C. below T_(S-B); saidconductive layer comprising said nanowires is applied to the surface ofthe binding layer of the substrate during the film manufacturing processand prior to the heat-setting step, wherein following deposition of thenanowires, the film is heated to temperature T₁; and said polymericsubstrate is a biaxially oriented polyester substrate.
 2. The processaccording to claim 1 wherein said nanowires are disposed on the exposedsurface of the binding layer by dispersing said nanowires in a liquidvehicle and coating the nanowire-containing liquid onto the exposedsurface of the binding layer.
 3. The process according to claim 1wherein the conductive film exhibits a total light transmittance overthe visible range (TLT) of at least 65%.
 4. The process according toclaim 1 wherein the conductive film exhibits a haze of no more than 50%.5. The process according to claim 1 wherein the polyester of the baselayer is selected from poly(ethylene terephthalate) and poly(ethylene2,6-naphthalate).
 6. The process according to claim 1 wherein thebinding layer is a copolyester.
 7. The process according to claim 6wherein the copolyester is selected from: (i) a copolyester derived fromethylene glycol, terephthalic acid and isophthalic acid; (ii) acopolyester derived from terephthalic acid, an aliphatic dicarboxylicacid and a glycol; and (iii) a copolyester derived from terephthalicacid, ethylene glycol and 1,4-cyclohexanedimethanol.
 8. The processaccording to claim 7 wherein the copolyester is a copolyester derivedfrom ethylene glycol, terephthalic acid and isophthalic acid whichexhibits a molar ratio of the terephthalic acid component to theisophthalic acid component in the range from 25:75 to 85:15.
 9. Theprocess according to claim 7 wherein the copolyester is a copolyesterderived from terephthalic acid, an aliphatic dicarboxylic acid andethylene glycol which exhibits a molar ratio of the terephthalic acidcomponent to the aliphatic dicarboxylic acid component about 50:50 toabout 70:30.
 10. The process according to claim 7 wherein thecopolyester is derived from terephthalic acid, azelaic acid and ethyleneglycol.
 11. The process according to claim 1 wherein the binding layerand base layer are coextruded.
 12. The process according to claim 1wherein the total thickness of the substrate is no more than 350 μm. 13.The process according to claim 1 wherein the sheet resistance of thetransparent conductive film is less than 100,000 ohms per square. 14.The process according to claim 1 wherein the nanowires are metalnanowires.
 15. The process according to claim 1 wherein the nanowiresare silver nanowires.
 16. The process according to claim 1 wherein thenanowires are carbon nanotubes.
 17. The process according to claim 1wherein the conductive layer is applied to the surface of the bindinglayer of the substrate between the two stages (longitudinal andtransverse) of a biaxial stretching operation.
 18. The process accordingto claim 1 wherein the heating temperature T₁ is in the range of fromabout 50° C. to about 240° C.
 19. The process according to claim 1wherein said binding layer is a heat-sealable layer.
 20. The processaccording to claim 6, wherein the copolyester is selected from the groupconsisting of: (A) a copolyester derived from one or more aliphaticglycol(s) and two or more aromatic dicarboxylic acids; (B) a copolyesterderived from at least one aromatic dicarboxylic acid and at least onealiphatic dicarboxylic acid with one or more glycol(s); and (C) acopolyester derived from an aliphatic diol and a cycloaliphatic diolwith one or more aromatic dicarboxylic acid(s).